The use of X-ray radiation in microscopy is known in the prior art. Such known X-ray microscopes typically are of the transmission mode type microscope, which include the source of X-rays on one side of the specimen or object to be viewed, and a radiation detector or imaging system on the other side of the object. Accordingly, only X-rays going through the material are detected. As a result, the use of such transmission type X-ray microscopes for viewing non-transparent or poorly transparent objects is limited, particularly with respect to soft X-ray lasers.
The technology related to X-ray microscopy has been greatly improved over the last ten years. Significant progress has been made in developing X-ray lasers, and synchrotron insertion devices, for providing increased brightness of laboratory X-ray sources.
Optical microscopy is limited to a resolution of about 3,000 angstroms, or about 0.3 micrometer. In lithographic systems used for inspecting integrated circuits, the capability for inspecting line widths of such circuits of less than 0.1 micrometers and below is now required. Electron microscopes can provide such resolution, but electron microscopes are slow to use for such inspection, and the electron beam can cause functional damage to the integrated circuit chip.
Skinner et al., in a paper entitled "Contact Microscopy With A Soft X-ray Laser", appearing in the Journal of Microscopy, Volume 159, part I, July, 1990, on pages 51 through 60, discusses the use of soft X-ray lasers for imaging live cells at high resolution, "thereby bridging the gap between electron microscope images of non-live cells that have undergone extensive specimen preparation, and low resolution but high fidelity images of live cells recorded with light microscopes." It is recognized that "to be of maximum utility to biologists, a soft X-ray laser contact microscope should be suitable for everyday use on fragile, living biological specimens." The paper also describes a system developed at Princeton University, Princeton, N.J., for generating soft X-ray laser beams. The installation of a contact microscope on a soft X-ray laser beam is shown. In this system, the soft X-ray laser beam is used to cause the shadow of a specimen to be recorded on photo-resist material.
Howells, et al., in a paper entitled "X-ray Microscopes", Scientific American, Volume 264, No. 2, February, 1991, pages 88 through 94, describes the state of X-ray microscopy, and the ongoing development of "soft" X-ray instruments, providing more than ten times better resolution than optical microscopes. It is indicated that "soft X-rays in the wavelength range of 20.0 to 40.0 angstroms (an angstrom is one ten-billionth of a meter), are sufficiently penetrating to image biological cells in many cases." The article goes on to describe the difficulty of focusing X-ray images and developments which led to the construction of a Fresnel zone plate for providing such focusing. Contact microradiography is described, as previously mentioned above, whereby an X-ray beam is passed through a sample to a resist (PMMA) material, causing a damage pattern in the resist material relating to details of the sample. Imaging X-ray microscopes are described whereby X-ray beams are passed through a condenser zone plate, for focusing the X-rays onto a region of a sample. The X-rays pass through the sample onto a micro-zone plate for focusing the X-rays into an image field that is picked up by a detector. A scanning X-ray microscope system is described whereby an X-ray beam is focused and scanned back and forth across a sample, with the scan beam being passed through the sample and detected by an X-ray counter.
A soft X-ray laser contact microscope system is described in Suckewer et al. U.S. Pat. No. 4,979,203. The described system uses an optical contrast microscope for inspecting and aligning a target prior to applying soft X-rays to the target for performing contact X-ray microscopy. An X-ray laser is used for providing the necessary soft X-ray beam.
Fields, et al. U.S. Pat. No. 3,702,933, entitled "Device and Method for Determining X-ray Reflection Efficiency of Optical Surfaces", teaches a method for determining the X-ray reflection efficiency and scattering characteristics of optical surfaces using much harder (shorter wavelength) X-ray radiation than the soft X-rays described above. As shown in the figure, X-rays of a known wavelength are generated by an X-ray source 11, and passed through a collimator 15, and therefrom passed through slits 17, for projection onto an area of a crystal monochromator 21 for diffracting the X-rays. The diffracted x-rays are then passed through slits 25, and projected onto a predetermined area of an optical test specimen 41. X-rays reflected off of the optical test specimen 27, are transmitted through a slit 33, and projected therefrom into an X-ray detector 31. The X-ray detector can be a Geiger-Muller counter. The intensity of the X-rays prior to and subsequent to reflection from the specimen 27 are compared for determining the efficiency of reflection of the optical surface of the test specimen 27. Suckewer et al. U.S. Pat. No. 4,771,430 shows an apparatus for enhancing soft X-ray lasing action through use of thin blade radiators in a target. Soft X-ray lasing action is generated in a defined plasma column. The plasma is produced by focusing a CO.sub.2 laser beam onto a carbon target. A magnetic field is used to compress the plasma into a thin column. A carbon disc in combination with carbon blades mounted perpendicular to the surface of the disc provides the target. The CO.sub.2 laser beam is directed to strike the surface of the disc for forming a plasma column. The column is cooled by radiation losses and heat conduction to the blade. The resulting soft X-rays are transmitted through a slot in the carbon disc.
Rocca U.S. Pat. No. 4,937,832 shows a method and apparatus for producing a soft X-ray laser beam in a capillary discharge plasma. As shown in FIGS. 1 and 2 thereof, the apparatus includes a pair of electrodes having axially oriented poles, respectively, for facilitating the exit of laser radiation. The electrodes are connected to a discharge circuit that includes a relatively large capacitor that is first charged to a given level, and then discharged across the electrodes. It is indicated that the power source can also be an electrical transmission line having a low impedance. Also discussed is the use of a high intensity magnetic field for containing the plasma.
Another Suckewer U.S. Pat. No. 4,704,718, similar to the above-described Suckewer patent, teaches the creation of a plasma column by focusing a CO.sub.2 laser pulse on a carbon target. A magnetic field is used to contain the plasma.
There are many other examples of soft X-ray generators and/or ultraviolet X-ray generators in the art. Other such references include U.S. Pat. Nos. 4,555,787, and 3,956,711.
Research is ongoing for providing improved sources of optics at short wavelengths, particularly wavelengths in the soft X-ray region from 1.0 nanometer to 30.0 nanometers wavelength. As previously indicated, laser sources and synchrotrons have provided soft X-rays for use in transmission X-ray microscopy. X-ray lasers provide a very high flux of short wavelength photons in very short pulses in single lines, whereas synchrotrons provide continuously tunable radiation. These sources of X-ray radiation tend to complement one another.
The present inventors recognized that by developing a reflection X-ray laser microscope, a step forward can be made in the field of the art for using such an apparatus in the field of lithography for inspecting integrated circuits, for example, and/or in the medical fields for analyzing biological specimens. They further recognized that the reflection coefficient for a number of biological materials is significant and differs substantially at soft X-ray wavelengths. They recognized that with a high flux of radiation from a soft X-ray laser, and through use of high sensitivity CCD (charge coupled device) detectors, a very compact reflection soft X-ray microscope can be provided. The inventors expect that such a microscope will provide magnification up to .times.100 with the resulting images recorded on CCD-array detectors, for example, having a pixel size in the order of 5.0 microns. Such a soft X-ray reflecting microscope is expected to have a resolution in the range of 0.05 microns.
It is further expected a soft X-ray laser source for use with the reflection X-ray microscope of the present invention will operate at a wavelengths of 18.2 nanometers (nm) with a beam energy of 1.0 to 3.0 mJ (millijoule) in a 10.0 to 30.0 nanosecond pulse. The laser can operate at shorter wavelengths (15.4 nm and 12.9 nm) with lower beam energy. A pumping CO.sub.2 laser pulse with an energy of 300.0 to 500.0 joules in 50.0 to 75.0 nanoseconds will create a lasing medium (plasma column) in a strong, magnetic field. Also, the repetition rate of the X-ray laser is three minutes.
The present inventors are scientists at the Princeton University Plasma Physics Laboratory (PPPL) (Skinner and Suckewer) and Mechanical and Aerospace Engineering Department (Suckewer) and Princeton X-ray Laser Inc. (Rosser), in Princeton, N.J., where soft X-ray laser development has been pursued for a number of years. In the March, 1987, issue of the "PPPL Digest", published by the Information and Administrative Services, Princeton Plasma Physics Laboratory, Princeton, N.J., the basics of soft X-ray technology existing in 1987 and various experimental results are discussed. In a later May, 1989 issue of the "PPPL Digest", a description of X-ray laser microscopy research at that facility is described. A composite X-ray laser microscope is shown and described for soft X-ray laser contact microscopy. Also, the basic principals of a soft X-ray laser are shown and described.